Often, magnetic systems are used to determine the position of the rotor of such an electrical machine. The magnetic system comprises a permanent magnet mounted on the rotor axis which acts as a rotary angle indicator and at least two sensors which detect the magnetic field generated by the rotary angle indicator. The angular position of the rotary angle indicator is determined from the sensor signals.
Both dipolar magnets and higher multipole magnets are used as permanent magnets. The absolute position of the rotation angle indicator can be determined with a dipole magnet, which is no longer the case with a higher multipole magnet. The loss of the information about the absolute position results in a gain in resolution with higher multipolar magnets.
In an electric motor as well as in a generator, there are inherently scatter and interference field sources which impair the measuring accuracy. In particular, in a structure in which the sensors are arranged in the region of the magnetic field generated by the stator, the stator is a strong source of interference fields due to its proximity to the rotary angle indicator. In many types of motors, the stator coils are operated with rotary current, whereby the magnetic field thereof also rotates about the motor axis.
In order to increase performance, electromotors are often operated close to or in saturation of the magnetic material of the stator and/or the rotor. This leads to field distortions and generates sensor signals with harmonic waves, i.e., the signals not only reflect the periodicity of the pole pitch of the rotor and stator as a function of the angle of rotation, but also contain superpositions of multiples of the corresponding fundamental waves. These field distortions significantly reduce the accuracy or even make the determination of the position impossible.
Electromotors are widely used in industrial, household and automotive electronics markets. Electromotors include all types, such as flat motors, synchronous motors, asynchronous motors, alternating current motors and stepping motors.
One example is the use as a drive motor of hybrid or purely electric vehicles. In order to guarantee a high efficiency and good driving comfort, the rotor position must be measured very precisely. Often, inductive or optical sensor systems are used for position detection, which however occupy a large installation space and are expensive.